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Thread: Weird $OD Spark Advanced FIXED, sorta

  1. #1
    Super Moderator dave w's Avatar
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    Weird $OD Spark Advanced FIXED, sorta

    I don't know what is mechanically wrong with the engine, EVERYTHING is new including the engine. Data log showed weird spark advance numbers, see screen shot below. Turns out the weird numbers in TunerPro RT is because the spark advance is a negative number ( used a timing light with a dial to confirm negative advance).

    Anyway, I disabled all spark retard programming in the $OD PROM Chip. Now the data log shows normal spark advance. Truck runs much better with spark retard disabled!

    I'm open for suggestions as why or what causes the spark advance to go negative. I define negative spark advance as degrees AFTER TOP DEAD CENTER. In this case, the negative spark advance was 5 degrees ATDC. Typically spark advance is degrees BEFORE TOP DEAD CENTER.

    Tried different computer, same result negative spark.

    Tried different Memcal, same result negative spark.

    Knock sensor measured 4K ohms. Knock sensor wire, computer to connector measured 0.6 ohms.

    Tried different (KNOWN GOOD) GM ignition module, same result negative spark.

    Tried different coil (stock GM), same result negative spark.

    Disable ERG in PROM, same result negative spark.

    Tried different MAP sensor, same result negative spark.

    Tried different CTS, same result negative spark.

    Weird, weird, weird problem!

    dave w
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  2. #2
    Fuel Injected! woody80z28's Avatar
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    Wow, that is really weird. I've never seen that before.

    What is it in, truck? Just curious because it doesn't look like a lot of spark. I just datalogged mine today for the first time in forever because I'm getting a Code 43, but only spark retard at WOT...pulling 5-8* and it feels super slow...
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  3. #3
    Super Moderator dave w's Avatar
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    It's bone stock 350 TBI re-manufactured engine. BCC is BJYL.

    dave w

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    could the wires be reversed in the dist on the magnitc pickup, i ran into that 1 time

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    1) I think you've figured this part out, but the "wierd" numbers are only Tunerpro's ADX interpretation of the 16 bit value which represents spark advance. Without telling TP that you have a signed integer the values are being interpreted as unsigned values with a maximum positive limit of 65,535 before the conversion formula.

    2) In Tunerpro I'm seeing BJYL with a maximum spark retard value of -9.8 degrees. (Yes, this is a double negative. This should be minimum spark advance). If datalogging revealed spark values no greater than 5 degrees ATDC then the spark advance was within the factory limits.

    So the questions are:
    1) Does the displayed spark advance match the actual spark advance? A: It appears that it does.
    2) Is the pcm indicating spark retard?
    3) Is the knock sensor detecting knock?
    4) If the knock sensor is replaced with a resistor, will the spark values return to normal?

    I have experience also with the wrong pickup coil (similar to wires being reversed) and have seen spark advance do the opposite of what is expected. But as I remember the displayed advance did not match actual. Was the distributor changed when the engine was replaced?

  6. #6
    Super Moderator dave w's Avatar
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    The engine seemed to be running fine for several months after the engine was replaced, then the engine experienced a loss of power symptom

    The distributor was replaced, but the engine still had a loss of power symptom.

    The attached 3-22-18_01.xdl is the first $OD data log I recorded when the pickup arrived for me to look at. This data log doesn't show any real significant knock retard.

    The vehicle owner is OK, with the "custom" chip, he needs his pickup on the road.

    I will try a resistor instead of the knock sensor, at a later date, the pickup is not currently available for me to work on.

    I'm hoping to get a few other ideas to try, before asking to work on the pickup again.

    The attached 4-22-18_01.xdl is the most recent data log, before the vehicle owner accepted delivery with the "custom" chip.

    dave w
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    Here are two theories to investigate:

    Viewing 3-22-18_01.XDL I see a restart at approximately 4:40.
    At 4:40:716 I see knock counts increasing and knock retard of approximately 1.8 deg.
    At 4:43:112 all traces of spark knock appear to be gone, and spark timing is significantly retarded.

    GM released calibrations for 92 - 95 vehicles intended to reduce cold startup spark knock. You can read a copy of one of the service bulletins here. Installing the cold knock fix calibration was one step in a matrix which included potentially changing oil filter type and replacing main bearing inserts. BJYL is one of the calibrations which was replaced. As I remember, shutting the engine down when warm would allow oil to drain out of the journals. The spark advance delivered during restart was enough to hammer at the bearings for a short time until oil pressure supported the crank. Over time this would cause excess clearance to develop in the mains. I wonder if it is possible that this truck's engine may be experiencing the same problem, and the knock that's showing at startup is the result.

    But this doesn't answer the question about why spark is retarded when no knock is occurring. I would have to look at the code, but I wonder if the pcm will store a value akin to "block learn" for spark if it detects knock during startup. I have never heard of this happening but the theory could explain what's happening. I did scan for additional restarts in the 3-22-18_01.xdl file but I didn't see any. Possibly you will have additional log files to compare?

  8. #8
    Super Moderator dave w's Avatar
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    Quote Originally Posted by 1project2many View Post
    Here are two theories to investigate:

    Viewing 3-22-18_01.XDL I see a restart at approximately 4:40.
    At 4:40:716 I see knock counts increasing and knock retard of approximately 1.8 deg.
    At 4:43:112 all traces of spark knock appear to be gone, and spark timing is significantly retarded.

    GM released calibrations for 92 - 95 vehicles intended to reduce cold startup spark knock. You can read a copy of one of the service bulletins here. Installing the cold knock fix calibration was one step in a matrix which included potentially changing oil filter type and replacing main bearing inserts. BJYL is one of the calibrations which was replaced. As I remember, shutting the engine down when warm would allow oil to drain out of the journals. The spark advance delivered during restart was enough to hammer at the bearings for a short time until oil pressure supported the crank. Over time this would cause excess clearance to develop in the mains. I wonder if it is possible that this truck's engine may be experiencing the same problem, and the knock that's showing at startup is the result.

    But this doesn't answer the question about why spark is retarded when no knock is occurring. I would have to look at the code, but I wonder if the pcm will store a value akin to "block learn" for spark if it detects knock during startup. I have never heard of this happening but the theory could explain what's happening. I did scan for additional restarts in the 3-22-18_01.xdl file but I didn't see any. Possibly you will have additional log files to compare?
    I really appreciate the theories.

    The vehicle stalled an died with modest throttle acceleration, that's why the restarts.

    The only log files I have other than the 3-22-18_01 have programming changes that mask the problem, like minimum temperature for knock retard.

    Wish I could have some logs when things were working OK.

    The second computer that was tried, had been on the shelf for several months so I don't think a learn would stick that long without power.

    dave w

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    The second computer that was tried, had been on the shelf for several months so I don't think a learn would stick that long without power.
    I agree. I am thinking of a learned value determined at each startup based on whether or not knock is detected. This could be tested in another vehicle by establishing a baseline at startup with no knock then simulating knock during a subsequent restart.

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    i had a simalar issue recently, ended up being the new balancer had the timing mark in the wrong spot due to a timing cover change. new balancer was at 12:00 old one was over on the side like an old school timing tab... i was starting out with seriously negative timing instead of 0 degrees......

  11. #11
    Super Moderator dave w's Avatar
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    Quote Originally Posted by skandolis View Post
    i had a simalar issue recently, ended up being the new balancer had the timing mark in the wrong spot due to a timing cover change. new balancer was at 12:00 old one was over on the side like an old school timing tab... i was starting out with seriously negative timing instead of 0 degrees......
    Thanks for the post. I didn't even think about the harmonic balancer timing indicator could be inaccurate. If, or when the truck is available to me again, I will put a degree wheel on the harmonic balancer and verify TDC.

    dave w

  12. #12
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    My guess is there's a spark bias scaler set at 0 instead of 20

  13. #13
    Super Moderator dave w's Avatar
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    Quote Originally Posted by delcowizzid View Post
    My guess is there's a spark bias scaler set at 0 instead of 20
    That's an interesting thought. I tested with a stock Memcal. I even downloaded the same BCC from this site, found Zero differences.

    dave w

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    sometimes i have to put a,b and so on in the file i started with my files get corrupted some times for no reason?

  15. #15
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    Can you add a link to the files or add the xdf adx and the stock bin here that you are using

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